Cardiovascular and respiratory responses to microinjection of L-glutamate into the caudal pressor area in conscious and anesthetized rats

The brainstem network controlling blood pressure: an important role for pressor sites in the caudal medulla and cervical spinal cord

: Although medullary control of blood pressure (BP) has been extensively studied, the contribution of critical regions, such as pressor sites in the caudal medulla and upper cervical spinal cord and the lateral tegmental field, remains controversial and underappreciated. A series of pressor sites caudal to the caudal ventrolateral medulla (CVLM), including the caudal pressor area (CPA) and medullocervical pressor area, play an important role in control of BP. Activation and inhibition of these sites elicits pressor and depressor responses, respectively. Basal sympathetic tone is provided principally by the medullary lateral tegmental field and rostral ventrolateral medulla (RVLM). RVLM presympathetic neurons, which project to and drive preganglionic sympathetic somata in the intermediolateral cell column, are powerfully regulated by neurons in CVLM via tonic and phasic inhibition. The current state of knowledge is summarized thus: rostrocaudally organized columns of pressor sites caudal to CVLM extend to the upper cervical spinal cord; CPA pressor responses are RVLM-dependent; CPA mediates pressor responses by (first) inhibiting RVLM-projecting inhibitory CVLM units and (second) activating RVLM-projecting excitatory CVLM units; the chemoreflex is CPA-dependent; the baroreflex is CPA-independent; pressor responses to raphe obscurus stimulation are CPA-dependent; and medullocervical pressor area pressor responses are RVLM-independent, likely mediated by direct projections to the intermediolateral cell column. In this review, we seek to underscore and characterize the critical role played by the caudal medulla and upper cervical spinal cord in BP regulation and highlight important gaps in knowledge in interactions between the caudal medulla and other regions controlling BP, which may prove critical in revealing central mechanisms underlying pathophysiology of, and pharmacotherapeutic targets for, hypertension.

Maps of cardiovascular and respiratory regions of rat ventral medulla: focus on the caudal medulla

The ventral medulla oblongata is critical for cardiorespiratory regulation. Here we review previous literature relating to sites within the ventral medulla that have been identified as having a 'cardiovascular' or 'respiratory' function. Together with the maps generated here, of sites from which cardiovascular and respiratory responses were evoked by glutamate microinjection, specific 'cardiovascular' regions have been defined and delineated. Commonly investigated regions, including the vasopressor rostral ventrolateral medulla (RVLM) and vasodepressor caudal ventrolateral medulla (CVLM), or areas only described by others, such as the medullary cerebral vasodilator area, are included for completeness. Emphasis is given to the caudal medulla, where three pressor regions, the caudal pressor area (CPA), the intermediate pressor area (IPA) and the medullo-cervical pressor area (MCPA), caudal to the vasodepressor CVLM were defined in the original data provided. The IPA is most responsive under pentobarbitone rather than urethane anaesthesia clearly delineating it from both the rostrally located CPA and the caudally located MCPA. The description of these multiple pressor areas appears to clarify the confusion that surrounds the identification of the 'CPA'. Also noted is a vasopressor region adjacent to the vasodepressor CVLM. Apart from the well described ventral respiratory column, a region medial to the pre-Bötzinger is described, from which increases in both phrenic nerve frequency and amplitude were evoked. Limitations associated with the technique of glutamate microinjection to define functionally specific regions are discussed. Particular effort has been made to define and delineate the regions with respect to ventrally located anatomical landmarks rather than the commonly used ventral surface or dorsal landmarks such as the obex or calamus scriptorius that may vary with the brain orientation or histological processing. This should ensure that a region can easily be defined by all investigators. Study of defined regions will help expedite the identification of the role of the multiple cell groups with diverse neurotransmitter complements that exist even within each of the regions described, in coordinating the delivery of oxygenated blood to the tissues.

Differential baroreceptor modulation mediated by the ventrolateral medulla

Previous studies have shown that pharmacological stimulation of a region denominated caudal pressor area (CPA), located in the caudal end of the ventrolateral medulla, induces increases in arterial blood pressure (BP). The aim of this study was to compare the responses on renal sympathetic nerve activity (rSNA) and BP responses mediated by stimulation of CPA or rostral ventrolateral medulla (RVLM), in intact or sino-aortic barodenervated rats. Male Wistar rats (300-350 g, n=15) were anesthetized (urethane 1.2 to 1.4 g/kg, i.v.) and artificially ventilated. The mean arterial pressure (MAP) and rSNA were measured during bilateral glutamate microinjection (10 nmo/100 nl) into the CPA or into the RVLM. Glutamatergic stimulation of the RVLM increased MAP (46+/-7 mm Hg) and rSNA (82+/-21%); during CPA stimulation, MAP and rSNA increased 60+/-7 mm Hg and 93+/-9%, respectively. However, despite the similarity of responses mediated by both regions, the duration of rSNA and blood pressure responses mediated by the CPA were significantly longer than the duration of the responses mediated by the RVLM. After barodenervation, there was an increase in the time-course and magnitude of sympathetic response only in response to RVLM stimulation but not in response to CPA. The results suggest a differential baroreceptor modulation on rSNA mediated by the ventrolateral medulla neurons. Glutamatergic activation of CPA neurons can cause large increases in the rSNA and BP with a weaker baroreceptor modulation when compared to responses mediated by the RVLM neurons.

Functional mapping of the cardiorespiratory effects of dorsal and median raphe nuclei in the rat

The dorsal (DRN) and median (MRN) raphe nuclei are important sources of serotonergic innervation to the forebrain, projecting to sites involved in cardiovascular regulation. These nuclei have been mapped using electrical stimulation, which has the limitation of stimulating fibers of passage. The present study maps these areas with chemical stimulation, investigating their influence on cardiorespiratory parameters. Urethane-anesthetized (1.2 g/kg, iv) male Wistar rats (280-300 g) were instrumented for pulsatile and mean blood pressure (MBP), heart rate, renal nerve activity, and respiratory frequency recordings. Microinjections of L-glutamate (0.18 M, 50-100 nl with 1% Pontamine Sky Blue) were performed within the DRN or the MRN with glass micropipettes. At the end of the experiments the sites of microinjection were identified. The majority of sites within the MRN (86.1%) and DRN (85.4%) evoked pressor responses when stimulated (DRN: DeltaMBP = +14.7 +/- 1.2; MRN: DeltaMBP = +13.6 +/- 1.3 mmHg). The changes in renal nerve activity and respiratory rate caused by L-glutamate were +45 +/- 11 and +42 +/- 9% (DRN; P < 0.05%), +40 +/- 10 and +29 +/- 7% (MRN, P < 0.05), respectively. No significant changes were observed in saline-microinjected animals. This study shows that: a) the blood pressure increases previously observed by electrical stimulation within the raphe are due to activation of local neurons, b) this pressor effect is due to sympathoexcitation because the stimulation increased renal sympathetic activity but did not produce tachycardia, and c) the stimulation of cell bodies in these nuclei also increases the respiratory rate.

Rate-dependent behavioral effects of stimulation of central motoric alpha(1)-adrenoceptors: hypothesized relation to depolarization blockade

AIM

The purpose of this review is to clarify how central alpha(1)-adrenoceptors control behavioral activity under varying conditions of activity and stress.

METHOD

The literature is reviewed regarding the behavioral actions of alpha(1)-agonists and antagonists, and alpha(2)-agonists and antagonists under conditions of high and low baseline activity and stress.

RESULTS

It was found that alpha(1)-receptor stimulation of active behavior has a number of similarities to rate dependency including: (1) a dependence on low-active, low-stress conditions or on the prior depletion of endogenous brain catecholamines; (2) a nonmonotonic dose-response relationship with high doses producing a fall-off or actual depression of activity; (3) a failure to be blocked at high agonist doses by alpha(1)-antagonists; and (4) a facilitation by alpha(2)-adrenoceptor agonists which produce an opposing hyperpolarization.

DISCUSSION

To explain these findings, it is proposed that high levels of stimulation of central alpha(1)-receptors produce, in host neurons, a depolarization block that impedes nerve impulse generation and inhibits active behavior. This effect is assumed to be precluded or mitigated by low-active, low-stress conditions, depletion of brain catecholamines, and by hyperpolarizing alpha(2)-agonists, and to be reversed at high agonist doses by alpha(1)-antagonists.

CONCLUSION

Because brain alpha(1)-receptors are not only involved in motor activity but also in the mechanism of action of antidepressant and stimulant drugs, arousal, anxiety, stress and psychosis, a depolarization block from intense stimulation of these receptors could have broad psychopharmacological consequences and underlie rate dependency to a variety of stimulant drugs.